The present invention relates to methods for producing higher esters of carboxylic acids by an organotin catalyzed transesterification reaction between lower alkyl esters of the carboxylic acid and alcohols and polyols. In particular, the present invention relates to novel methods for forming heretofore unattainable 1,2- and 1,3-polyol esters by organotin catalyzed transesterification. In addition, the present invention relates to methods for synthesizing such polyol esters, as well as other polyol and alcohol esters, by organotin catalyzed transesterification, and recovering the resulting esters substantially free of the organotin catalyst.
Esters of unsaturated carboxylic acids and of aromatic polycarboxylic acids are of increasing commercial importance as polymerizable monomers. Materials of this nature are used to form both homopolymers and copolymers; which have commercial uses in many applications. Such applications include coatings for paper products, waste water treatment systems, optical lens coatings, floor polishes, anaerobic adhesives, pour point depressants, paper coatings, UV and EB coatings and adhesives, textile finishes, pressure sensitive adhesives, viscosity index improvers, potting compounds and sealants, photopolymers for electronics and printing plates, rubber and plastics modifiers, UV curable inks and overprint varnishes, dental and medical resins, reactive diluents for radiation curable oligomers, crosslinkers for rubber vulcanization, moisture barrier films, ion exchange resins, PVC plastisols, encapsulation and impregnation of small diameter spheres, leather finishes, binder resins for sand castings, UV curable resins for imaging systems, silane intermediates, and the like; such applications being well known to those skilled in the art.
One group of monomers of particular interest are the polyfunctional monomers; that is to say, esters of unsaturated carboxylic acids with polyfunctional alcohols. As is also well known to those skilled in the art, materials of this nature can be used as cross-linking agents to form rigid coatings which are insoluble in normally-used solvents. Of particular interest are the esters of acrylic acid (2-propenoic acid) and methacrylic acid (2-methyl-2-propenoic acid). These esters, both monofunctional and polyfunctional, have long been used as components of homopolymers and/or copolymers for the applications described above.
Another group of monomers of particular interest are the unsaturated esters of aromatic polycarboxylic acids. The polymerization products of such monomers possess excellent dielectric properties, dimensional stability, heat resistance, weatherproofness, solvent resistance and mechanical properties. Preferred polymer products also possess optimum optical properties, including transparency, refractive index and surface hardness. Such polymers are desirable for use as optical materials.
In the past, as in current industrial practice, the above monomers have been made by direct esterification, i.e., the acid catalyzed reaction of an unsaturated carboxylic acid with a mono- or polyhydric alcohol. The major exception to this procedure is the preparation of unsaturated esters containing a basic functional group, such as an amine group. In these cases, the products have traditionally been made by a transesterification procedure, using catalysts such as sodium methylate, lead oxide, tetraisopropyl titanate, and the like. (See, e.g., U.S. Pat. No. 3,642,877.) In the commercial preparation of compounds of this type, the final reaction mixture is subjected to fractional distillation under reduced pressure, in order to obtain the desired monomer in a state of high purity, free of the metallic catalyst and/or excess polymerization inhibitor, which must be present during the preparation of these compounds.
By contrast, the products of the acid-catalyzed direct esterification are purified by base-washing procedures, which will remove acid catalyst and excess unreacted carboxylic acid as well as excess polymerization inhibitors. Although, in principle, it would be possible also to purify such reaction products by fractional distillation under reduced pressure, in industrial practice this procedure is only used with materials of relatively high volatility. This is because many of these products, particularly the esters of long-chain aliphatic alcohols as well as the esters of polyhydric alcohols, have relatively high boiling points, even when high vacuum is employed. In industrial practice it is very difficult to attain pressures less than about 1 mm Hg (more usually the vacuum used varies from about 10 to 20 mm Hg); and even under these conditions, the boiling points of these esters are so high as to make them very difficult to distill. As is well known in the art, monomers of this nature will tend to polymerize at temperatures in excess of about 115.degree.-120.degree. C., even when inhibited with various polymerization inhibitors. Consequently, in industrial practice, it is preferred to isolate the reaction products as "bottoms" products, which are not distilled.
The acid-catalyzed direct esterification described above suffers from various disadvantages, particularly the occurrence of several side reactions. In particular, such processes may cause the formation of color bodies which may be difficult, if not impossible, to remove from the finished product. Such color bodies may render the product unsuitable for many industrial applications, in particular in areas such as paper treatment chemicals, industrial coatings and the like. Also, the acid-catalyzed side reactions will lead to the production of by-products. Such by-products, although not necessarily deleterious in themselves, act as unreactive diluents for the final product and thus reduce its efficacy. Other disadvantages include the need to use an excess of the carboxylic acid to complete the reaction. This excess carboxylic acid cannot generally be recovered and recycled; and therefore represents an extra raw material cost as well as an increased waste disposal cost.
It is, of course, possible to prepare many of these products by transesterification, but many of the same disadvantages will remain. In particular, many potential transesterification catalysts such as aluminum isopropoxide, sodium methoxide, tetraisopropyl titanate and lead oxide, also catalyze the same side reactions described above. A further disadvantage is that many of these catalysts are difficult, if not impossible, to remove from the finished product, especially on an industrial scale.
Metal-containing catalysts, such as tetraisopropyl titanate, aluminum isopropoxide or dibutyltin oxide, can be used as catalysts for transesterification reactions of monofunctional (monohydric) alcohols, as well as of polyhydric alcohols in which the hydroxyl groups are not in close proximity. However, in the case of vicinal polyols, such as ethylene glycol, 1,2-propanediol and glycerol; or in the case of polyols where the hydroxyl groups occupy 1,3-positions, such as 1,3-propanediol, trimethylolpropane, or 1,3-butanediol; metallic catalysts, such as the ones mentioned above, form, respectively, five or six-membered metal-containing cyclic compounds. These cyclic compounds are relatively unreactive and will participate only slightly, if at all, in the catalytic reaction steps needed to bring the transesterification reaction about in a reasonable length of time. It is, therefore, not feasible to use these materials as catalysts for the preparation of esters derived from the polyhydric alcohols described above.
Another metal-containing catalyst system made from dialkyltin dichlorides has recently been reported. Otera et al., J. Org. Chem., 54, 4013-14 (1989) discloses that dialkyltin dichlorides form a stable, rigid, ladder structure (with four tin atoms), which functions as a template that exercises steric control during transesterification. These materials have been described as "reverse micelles" whose structure has to remain intact in order to be catalytic. Hydrocarbon solvents are preferred, because polar solvents dissociate the complex, leading to poor catalytic activity. However, predominantly non-polar reaction solvents are undesirable for the transesterification of commercially desirable carboxylic acid monomers. In addition, this reference contains no disclosure regarding how to isolate the pure product ester, a step which is essential to commercial manufacture. Furthermore, these compounds are reported to hydrolyze and form tetraorgano distannoxanes.
Otera et al., J. Org. Chem., 56(18), 5307-11 (1991) disclose these compounds to be effective catalysts in the transesterification of monohydric alcohols. This is confirmed by Otera et al., Tetrahedron Lett., 27(21), 2383-6 (1986), which also discloses the transesterification of diols other than 1,2- and 1,3-diols.
U.S. Pat. No. 4,473,702 discloses the synthesis of a diallyl ester of an aromatic dicarboxylic acid by transesterification of a dialkyl ester of an aromatic dicarboxylic acid with allyl alcohol. The reaction is catalyzed by a dialkyltin dichloride, dialkyltin oxide or mixtures thereof in combination with a second catalyst such as metallic magnesium, zinc, tin, lead, aluminum, nickel or zirconium, or oxides thereof. The disclosure of this patent is limited to reactions employing monohydric alcohols and the resulting ester is separated by conventional distillation and recrystallization methods, with no indication that the ester is obtained in a pure form free of the metal catalyst.
None of the foregoing publications discloses a transesterification catalyst or method that will allow for transesterification of 1,2- and 1,3-polyols, or the isolation of pure product ester. There remains a need for a catalyst system effective in the transesterification of 1,2- and 1,3-polyols. A system that would permit the isolation of the pure ester product free of the metal catalyst would be even more desirable.